The present invention relates to a process and apparatus for injecting gas, particularly a purge or scavenging gas, into an outlet opening of a metallurgical vessel. More particularly, the present invention is directed to such a process and apparatus wherein a movable plate of a sliding closure unit moves between open and closed positions to control the discharge of molten metal through the outlet opening, and wherein when the movable plate is in the closed position a flow of purge or scavenging gas is supplied from a gas supply system through the area of the movable plate confronting the outlet opening into the outlet opening. It is known to employ such an arrangement to guard against the solidification or freezing of the molten metal or the formation of lumps of metal within the outlet opening, for example due to the molten metal being cooled by the walls of the outlet opening, or at least to maintain the molten metal fluid until the movable plate of the sliding closure unit next is moved to the open position, thereby to attempt to ensure full flow discharge of the molten metal. However, with known systems it has not always been possible to ensure that full flow discharge of the molten metal will occur upon moving the movable plate to the open position. Particularly, if the movable plate has been in the closed position for a substantial amount of time, the outlet opening becomes clogged or choked with solidified metal and must be burned free, for example with an oxygen lance. This process is complicated, time consuming and expensive.
West German Patent No. 3,506,426, corresponding to French Patent No. 2,577,828 and to U.S. patent application Ser. No. 831,917, filed Feb. 21, 1986, discloses a process for preventing metal deposits from forming in the outlet opening during discharge of molten metal through the outlet opening. For this purpose, there is provided both a continuous flow and also a pulse-like injection of gas, particularly an inert gas. These two gas flow injections are controlled to maintain the outlet opening clear.